Explosive stimulation well completions

ABSTRACT

Disclosed is a method and apparatus for preventing damage to the tubular goods and cement sheath of wells during an explosive stimulation treatment. The invention includes the use of periodic intervals of a shock absorbing medium in the cement sheath of a well bore to absorb the damaging shock energy emanating from explosions, used to stimulate producing zones, by the placement of the shock absorbing medium in the cement sheath above and below the wellbore zone to be stimulated.

Uiteil States atent [191 Land 1 Jan. 2, 1973 54 EXPLOSHVE STIMULATION WELL 3,277,962 10/1966 Flickinger et al ..166/285 x COMPLETIONS 3,534,816 10/1970 Showalter ..166/285 5] Immntor: a M d, Tulsa k a 3,587,744 6/1971 Spencer ..l66/299 [73] Assignee: Cities Service Company, Tulsa, Primary Examiner-Verlin R. Pendegrass Okla. Attorney-J. Richard Geaman [22] Filed: Dec. 11, 1970 ABSTRACT 21 A LN 97 202 1 pp 0 Disclosed is a method and apparatus for preventing damage to the tubular goods and cement sheath of [52] US. Cl ..102/20 wells during an explgsive stimulation treatment. The

invention includes the use of periodic intervals of a [58] Flam of Search "102/20, 21-6; 166/63 shock absorbing medium in the cement sheath of a 166/289 299 well bore to absorb the damaging shock energy emanating from explosions, used to stimulate produc- [56] References cued ing zones, by the placement of the shock absorbing UNITED STATES PATENTS medium in the cement sheath above and below the wellbore zone to be stimulated. 2,921,519 1/1960 Martin ..l02/2l.6 X

3,075,463 1/1963 Eilers et al ..l66/299 8 Claims, 1 Drawing Figure PMENTEU 2 I95 3. 7 07.914

KARL M. LAND,

INVENTOR.

BYXWW ATTORNEY.

EXPLOSIVE STIMULATION WELL COMPLETIONS BACKGROUND OF THE INVENTION damage to the tubular goods and cement sheath of 0 production wells during an explosive stimulation treatment.

In the primary and secondary production of natural gas, petroleum and minerals from subterranean reservoirs, often areas of low permeability are encountered. By permeability is meant the measure of the reservoirss capacity for transporting fluid through its pores. This capacity may be expressed in petroleum engineering units as darcies of millidarcies, a darcy being that capacity of a reservoir rock required to allow flow of one cubic centimeter of one centipoise viscosity fluid through one centimeter square of rock per second under a pressure gradient of one atmosphere. Therefore, a given increase in permeability will result in the capability of an increase in fluid flow through that portion of the reservoir rock treated. An increase in the permeability within a reservoir is called stimulation, which is a required treatment for low productivity reservoirs.

When a reservoir rock is fractured, there exists an increase in the rock permeability with the flow of the reservoir fluids allowed into new portions of the reservoir matrix which were previously impermeable to fluid flow. During reservoir stimulation, an increase in permeability is established and greater flow of reservoir fluid is afforded through the stimulated areas. The production interval of the hydrocarbon or mineral containing reservoir is generally composed of strata of varying permeability. The low permeability areas are formed of a tight, low permeability material through which fluid flow is restricted by the configuration of the rock matrix. Since few rock formations are homogeneous, the heterogeniety of the formation must be considered as a restraint on petroleum and natural gas production. It is desirable to treat the low permeability zones of a reservoir by stimulation, thereby affording a greater capacity through which reservoir fluids may flow so as to subject a greater volume of reservoir to hydrocarbon or mineral recovery.

The conventional methods available for the combating of the heterogeneity of reservoirs involve complex stimulation techniques, such as hydraulic fracturing, acidizing or explosively detonating the less permeable reservoir zones. One of the earliest stimulation techniques was by the use of liquid explosives, for example nitroglycerin, in a wellbore opposite the uncased portion of the producing formation to be stimulated. A severe problem exists with the explosive stimulation of mineral, oil and gas wells in that associated with the stimulation is the destruction of the casing and cement sheath and loss of the necessary isolating cement bonds and the destruction of the casing and formation interfaces in the wellbore. The distortion of tubular goods and the destruction of the cement bonds contained in wells stimulated by explosive fracturing prevent subsequent efficient production. Also, shock waves, generated by the explosive stimulation treatment,

propagate up the casing and cement sheath and destroy the cement seal. The propagation of the explosive shock waves causes undesirable pressure and fluid in the casing between the producing zones within the production wellbore.

Conventional practices in explosive stimulation treatments generally involve stemming the well above the treatment interval by using cement, sand, gravel, and viscous fluids or a combination of these materials to pack a well and isolate the explosive detonation and shock wave propagation. The stemming practice confines the explosion, but does not provide protection to the well completion. What is required is a method and apparatus for preventing damage to the tubular goods and cement sheath of wells during explosive stimulation treatments.

It is an object of this invention to provide apparatus for an improved reservoir and mineral deposit treatment technique.

It is another object of this invention to provide a method for the protection of the tubular goods and cement sheath in a well during explosive stimulation treatments.

It is still a further object of the present invention to provide apparatus by which the tubular goods contained with a wellbore may be protected during stimu lation by explosive fracturing.

With these and other objects in mind, the invention is hereinafter set forth with particular reference to the following drawing and description SUMMARY OF THE INVENTION The objects of the present invention are accomplished by apparatus and a method for the explosive stimulation of a wellbore. The apparatus comprises a first interval of shock absorbing medium contained within a cement sheath for a distance above the wellbore section to be explosively stimulated. A conventional cement sheath is completed within the wellbore section so as to traverse the wellbore section to be explosively stimulated. A second interval of shock absorbing medium contained with a cement sheath is completed for a distance below the wellbore interval to be explosively stimulated. Multiple completions of the apparatus of the present invention may be positioned for each zone in the wellbore to be explosively stimulated. The shock absorbing medium generally comprises a slurry of collapsible spheres contained within a conventional completion cement circulated in the well.

The apparatus for the explosive stimulation of a bottomhole section of a wellbore would comprise an interval of shock absorbing medium completed to a distance above the bottomhole section of the wellbore to be explosively stimulated. A conventional cement sheath is then completed within the wellbore section to be explosively stimulated.

The present invention also comprises a process for the explosive stimulation of a wellbore. The process comprises the completing and cementing of lengths of easing above and below the wellbore section to be explosively stimulated, with a conventional completion cement utilized so as to traverse the wellbore section to be explosively stimulated. The intervals above and below the wellbore section to be explosively stimulated are completed with the shock absorbing medium. The

conventional casing is then perforated and explosive introduced through the perforations into the wellbore section. Subsequently, the explosive is detonated to stimulate the wellbore section.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be more fully understood by referral to the following drawing in which a preferred embodiment is shown which represents the apparatus of the present invention for theexplosive stimulation of a wellbore utilizing completions of the shock absorbing medium.

DETAILED DESCRIPTION OF THE INVENTION The present invention discloses apparatus and a process for preventing damage to the tubular goods and cement sheath of a well during explosive stimulation of the well. The invention utilizes periodic intervals of a shock absorbing medium in the cement sheath to absorb the damaging shock energy emanating from explosions used to stimulate producing zones in a hydrocarbon or mineral bearing wellbore. In particular, the invention utilizes the placement of intervals of a shock absorbing medium in the cement sheath in the vicinity of zones to be explosively treated in order to protect the wellbore completions and allow adequate explosive stimulation of the producing zones.

The present invention may be more fully understood by referral to one embodiment of the invention, as depicted in the drawing, for isolating damaging forces resulting from explosive stimulation of an intermediate zone in a wellbore. Cement sheaths 101 and 103 containing shock absorbing medium having collapsible spheres 114 and 115 therein, respectively, and casing strings 107 and 113 contained therein, are used to position and complete joints 107 and 113 above and below a wellbore zone 112 to be treated with explosive. A first conventional casing joint 109 extends through a portion of the overburden rock 106, is completed to the wellbore by cement sheath 116 and is connected to casing joint 107, by cement sheath 101 containing collapsible spheres 114 and by casing connector 108. A second conventional casing string 104 traverses the wellbore zone 112, which will be explosively stimulated, and is connected to casing joint 107 by casing connector 118 and to casing joint 113 by casing connector I05. Casing string 104 is completed to the wellbore by cement sheath 102. Casing string 113, with cement sheath 103 having collapsible spheres 115 therein and in similar form is connected by casing connector 110 to a third conventional casing string 111 which is completed to the wellbore by cement sheath 117. The apparatus of the present invention may be repeated in like manner for as many productive zones in the wellbore which are to be stimulated by explosives.

The casing string arrangement described, may be cemented into place by either of two methods. In the first method, the conventional casing string 104 traversing the wellbore to be explosively stimulated may be perforated and the cement squeezed through the perforations into the upper annulus formed between the wellbore and casing joint 107 with an interval of shock absorbing medium introduced therewith. In a similar fashion, the cement may be circulated into the lower annulus. A second completion technique would involve squeezing the cement down the annulus between the casing and the wellbore with slugs of shock absorbing medium introduced so that the shock absorbing medium forms the cement sheath above and below the wellbore section to be explosively stimulated. Once in place, the shock absorbing medium would serve as a barrier against the damaging forces generated by the explosives.

The arrangement of the cement sheath having shock absorbing medium therein may be placed at more than one depth interval in the well should there exist multiple zones in the well to be explosively stimulated. A bottomholezone may also be isolated by placing one or more intervals of shock absorbing medium in the completion immediately above the bottomhole zone. A conventional cement sheath being completed to the bottom of the wellbore so as to traverse the wellbore zone of interest and complete the bottomhole of the well.

In the utilization of the apparatus of the present invention, the process for explosive stimulation of the wellbore involves the completing and cementing of casings above and below the wellbore sections to be explosively stimulated with shock absorbing medium. Generally, from about 10 to 20 feet of shock absorbing medium above and below each zone to be stimulated is required to provide the necessary shock absorbing media. A conventional cement sheath is positioned between the intervals of shock absorbing medium so as to traverse the wellbore section to be explosively stimulated. Subsequent to the completion and cementing of the casing at the various wellbore zones, the casing and conventional cement sheath are perforated at the wellbore zones to be stimulated. After perforating is completed, explosive is introduced into the wellbore sections to be explosively detonated, through the perforated casing joints. Either sequentially or simultaneously, the explosive loaded wellbore sections are detonated to stimulate the wellbore sections of interest.

By positioning the shock absorbing medium above and below the wellbore sections of interest, the shock waves generated by the explosive stimulation treatments do not propagate up the casing and cement sheath, thereby destroying the cement shield, but are absorbed by the destruction of the collapsible spheres contained within the shock absorbing medium. This phenomena is similar to the energy dissipation ofa projectile traversing a medium of tightly packed spheres in that the energy is absorbed by the collapsing spheres. The explosions shock waves are dissipated into the for mation rather than up the casing string, thereby preventing damage to the tubing goods and cement sheath of the well during explosive stimulation.

The shock absorbing medium utilized in the present invention may comprise any conventional wellbore completion cement consisting of from about 30 to about volume percent of a collapsible material and, in particular, spherical materials have proven to be excellent shock absorbers. Common collapsible spheres which may be utilized may be selected from the group consisting of glass spheres polymeric spheres, float ash cenospheres, metallic spheres and combinations thereof. The underlying characteristic of the collapsible materials utilized being their ability to absorb the energy generated by the explosive stimulation, exhibit corrosion resistance to contain the hydrocarbon fluids or mineral solutions produced from the formation, as well as yield a long service life for the production of the stimulated wellbores. Upon collapse of shock absorbing material, a tight network of honeycombed cement remains in the cement sheath to provide lasting support for the casing string and inhibition against fluid circulation in the annulus between the casing and wellbore walls.

By the use of the apparatus and process of the present invention, wellbore zones may be explosively stimulated and developed while protection is provided to the tubular goods and cement sheath contained therein. The process of the present invention presents a process by which the damage normally suffered by wellbore strings during explosive stimulation may be avoided while substantially increasing the effective drainage radius of the well. Therefore, by the use of the present invention it is possible to obtain stimulation in the vicinity of the wellbore desired while still retaining rigidity in those sections of the wellbore used for support of the production and casing strings.

While the present invention has been described herein with respect to particular embodiments thereof, it will be appreciated by those skilled in the art, however, that various changes and modifications can be made without departing from the scope of the invention.

Therefore, I claim:

11. Apparatus for the explosive stimulation of a wellbore, comprising:

a. a conventional cement completion of a casing string to a distance above the wellbore section to be explosively stimulated;

b. a first interval of shock absorbing medium completion of the casing string to the top of the wellbore section to be explosively stimulated, said shock absorbing medium comprising conventional completion cement containing from about thirty to about seventy volume percent collapsible material;

c. a conventional cement completion of the casing string through the wellbore section to be explosively stimulated; and

d. a second interval of said shock absorbing medium completion of the casing string below the wellbore section to be explosively stimulated.

2. The apparatus of claim 1 in which the shock absorbing medium comprises a conventional completion cement containing from about 30 to about volume percent collapsible spheres.

3. The apparatus of claim 1 further comprising multiple completions of the apparatus of steps (a) through (d) for each zone in the wellbore to be explosively stimulated.

4. The apparatus of claim 3 wherein the intervals of shock absorbing medium are from about 10 to about 20' feet.

5. Apparatus for the explosive stimulation of the bottomhole section ofa wellbore, comprising:

a. a conventional cement completion of a casing string to a distance above the bottom hole of the wellbore to be explosively stimulated;

b. an interval of shock absorbing medium completion of the casing string to the top of the bottomhole section, said shock absorbing medium comprising conventional completion cement containing from about thirty to about seventy volume percent collapsible material; and

c. a conventional cement completion of the casing string to the bottom of the well.

6. The apparatus of claim 5 in which the shock absorbing medium comprises a conventional completion cement containing from about 30 to about 70 volume percent collapsible spheres.

7. The apparatus of claim 6 wherein the interval of shock absorbing medium is from about 10 to about 20 feet.

8. A process for the explosive stimulation of a wellbore, comprising:

a. completing a casing string with one or more intervals of from about 10 feet to about 20 feet of shock absorbing medium having from about thirty to about seventy volume percent of collapsible spheres above and below the wellbore section to be explosively stimulated with a conventional cement completion of the casing string positioned so as to traverse the wellbore section to be explosively stimulated;

b. perforating the completed conventional casing joint;

0. introducing explosive into the wellbore section through the perforated casingjoint; and

d. detonating the explosive. 

1. Apparatus for the explosive stimulation of a wellbore, comprising: a. a conventional cement completion of a casing string to a distance above the wellbore section to be explosively stimulated; b. a first interval of shock absorbing medium completion of the casing string to the top of the wellbore section to be explosively stimulated, said shock absorbing medium comprising conventional completion cement containing from about thirty to about seventy volume percent collapsible material; c. a convenTional cement completion of the casing string through the wellbore section to be explosively stimulated; and d. a second interval of said shock absorbing medium completion of the casing string below the wellbore section to be explosively stimulated.
 2. The apparatus of claim 1 in which the shock absorbing medium comprises a conventional completion cement containing from about 30 to about 70 volume percent collapsible spheres.
 3. The apparatus of claim 1 further comprising multiple completions of the apparatus of steps (a) through (d) for each zone in the wellbore to be explosively stimulated.
 4. The apparatus of claim 3 wherein the intervals of shock absorbing medium are from about 10 to about 20 feet.
 5. Apparatus for the explosive stimulation of the bottomhole section of a wellbore, comprising: a. a conventional cement completion of a casing string to a distance above the bottom hole of the wellbore to be explosively stimulated; b. an interval of shock absorbing medium completion of the casing string to the top of the bottomhole section, said shock absorbing medium comprising conventional completion cement containing from about thirty to about seventy volume percent collapsible material; and c. a conventional cement completion of the casing string to the bottom of the well.
 6. The apparatus of claim 5 in which the shock absorbing medium comprises a conventional completion cement containing from about 30 to about 70 volume percent collapsible spheres.
 7. The apparatus of claim 6 wherein the interval of shock absorbing medium is from about 10 to about 20 feet.
 8. A process for the explosive stimulation of a wellbore, comprising: a. completing a casing string with one or more intervals of from about 10 feet to about 20 feet of shock absorbing medium having from about thirty to about seventy volume percent of collapsible spheres above and below the wellbore section to be explosively stimulated with a conventional cement completion of the casing string positioned so as to traverse the wellbore section to be explosively stimulated; b. perforating the completed conventional casing joint; c. introducing explosive into the wellbore section through the perforated casing joint; and d. detonating the explosive. 